"All plants make protease inhibitors. It's a natural mechanism to protect them against insects. We wanted to know whether we could use it in a biotechnology approach to protect cotton," says Anderson, of La Trobe University in Melbourne.

Anderson is also chief scientific officer of Hexima Limited, a company developing the new insecticidal genes.

Protein digestion affected

Previous research has found that the flowers of solanaceous plants, such as potato and tobacco, express high levels of particular chemicals that are toxic to pests.

The chemicals, called proteinase inhibitors (PIs), inhibit caterpillar gut enzymes, called proteases, used to digest protein.

For decades scientists have been trying to engineer PI genes into cotton in the hope of slowing down the rate at which caterpillars munch the plant, reducing the need for pesticide sprays.

But, says Anderson, previous research has found that while PIs initially slow down the damage to crops, the pests soon adapt.

Now, Anderson and team think they have discovered a way around this adaptation to make PI genes a more effective weapon against pests.

New protease genes expressed

The researchers took a closer look at what happened in the gut of caterpillars exposed to PIs from tobacco.

They found that over time an insect would adapt by producing new proteases that weren't affected by the tobacco PIs.

"It turned on a new set of genes that weren't affected by these proteinase inhibitors," says Anderson.

She and colleagues isolated these newly activated genes and worked out the chemical structure of the new proteases they were producing.

They then searched for PIs that would inhibit these particular proteases and found them in potato leaves.

Anderson says that when they fed a combination of the tobacco and potato PIs to caterpillars this was more effective at slowing down the pests, than previous research had found.

The researchers also genetically engineered cotton plants containing genes for both PIs to see how effective they would be in the field.

They compared the performance of ordinary cotton plants with those that had been engineered to contain just the tobacco PI gene, just the potato PI gene, and those containing both PI genes.

"We found that the plants that had the combination of both types of inhibitors performed much better than all the other plants and we had a better yield of cotton from those plants," says Anderson.

"It's not appropriate to just use one class of proteinase inhibitor. You have to be aware that insects can respond and make new proteases."

She says field trials carried out in Queensland showed the plants with both PIs increased cotton yields by about 21%, whereas previous field trials using PIs had found no improved yield.

Anderson says the aim is to combine both PI genes, patented by Hexima, with Monsanto-owned Bt genes, currently used in cotton.

She says one of the aims is to help reduce the risk of insects developing resistance to existing Bt cotton varieties.

Too early to call

CSIRO cotton biotechnologist, Dr Danny Llewellyn, says the "synergistic effects" between the two PIs in the new research is interesting.

But, he does not believe Anderson and colleagues have proven that the combined PIs are responsible for the increase in yield shown.

He says the researchers only showed an increase in yield in one line of transgenic cotton and this could have been due to mutations occurring in tissue culture.

"To prove that it's actually caused by the genes you've added in, generally you're required to have multiple different engineered plants that show the phenotype, but they've only got one," says Llewellyn.

He says one aspect of the research that raises questions is that the increase in yield occurred in a plant that didn't seem to express very high levels of the potato PI.

"The transgenic cotton they've produced has very little of the second inhibitor," says Llewllyn, who is working with Monsanto and other large companies to produce insect resistant cotton.

Anderson agrees the PI from potato leaves was not expressed very well in the transgenic cotton and her team hopes to tweak the plant so it will express this second PI at greater levels.